Cook-by-weight microwave oven

ABSTRACT

A microwave oven having four small holes in the floor of the cavity so that weight of food bodies within the cavity can be coupled by four nonconductive support columns protruding through the holes to a scale below the cavity. The scale may provide an analog signal which is a function of the weight. The signal may be converted to a digital input for a microprocessor which calculates the heating time for that weight over an input temperature range.

CROSS-REFERENCE TO RELATED CASES

This is a continuation of application Ser. No. 190,285, filed Sept. 24,1980, now abandoned.

BACKGROUND OF THE INVENTION

As is well known in the art, microwave ovens cook using a differentprinciple than conventional gas or electric ovens. More specifically, amicrowave field in a cavity is absorbed by a food body inducing heatthroughout the interior of the food body. Because of the differentprinciple of cooking, traditional techniques and recipes of specifyingtemperature and time for cooking are not applicable to microwave ovens.

Research and development has been performed to determine new ways tosimplify the task of the operator in determining how long to expose thefood to microwave energy. One possibility considered was to use theweight of the food to determine the heating time in the microwave oven.Many problems were encountered, one of which was to provide a choke fora scale which would sense the weight of the food within the microwavecavity providing an input to a microprocessor.

SUMMARY OF THE INVENTION

The invention discloses a microwave oven comprising an outer housing, aconductive cavity within the housing, a chamber below the cavity and inthe housing, a weight sensing device positioned in the chamber, aplurality of spaced apertures in the floor of the cavity communicatingto said chamber, said apertures having a perimeter of less thanone-quarter wavelength of said microwave energy, nonconductive verticalcolumns supported by the weight sensing device and protruding throughthe floor into the cavity, a plate positioned in the cavity andsupported by the columns whereby the weight of objects placed on theplate is coupled to the weight sensing device, said weight sensingdevice providing a first signal which is a function of the weightsupported by said vertical columns, a magnetron for supplying microwaveenergy to the cavity, a microprocessor, a control panel having anoperator selectable means for providing a second signal corresponding tothe initial temperature of the object to the microprocessor, and themicroprocessor in response to the first and second signals controllingthe magnetron. It may be preferable that the weight sensing device be acompliant scale. Also, it may be preferable that the nonconductivecolumns be transparent to microwave energy.

It may be preferable that in response to the first and second signals,the microprocessor calculate the exposure time to microwave energy toraise the temperature of the object from its initial temperature to apredetermined higher temperature. For example, the initial temperaturemay be refrigerator temperature at 40° F. and the predeterminedtemperature may be approximately 65° F. Also, the initial temperaturemay be approximately 65° F. and the predetermined temperature may beapproximately 160° F.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of preferred and alternateembodiments of the invention will be more easily understood withreference to the drawings wherein:

FIG. 1 is a front elevation partially cut away of a microwave oven usingthe invention to advantage;

FIG. 2 is a side elevation of the microwave oven of FIG. 1 taken alongline 2--2;

FIG. 3 is a top view of the microwave oven of FIG. 1 taken along line3--3;

FIG. 4 is an alternate embodiment of the compliant member, light sourceand light sensitive device of FIGS. 1, 2 and 3;

FIG. 5 is a block diagram of a microwave oven system embodying theinvention;

FIG. 6 is an expanded view of the control panel of the microwave oven ofFIG. 1;

FIG. 7 is a software flow diagram of the programming of a microwave ovenembodying the invention;

FIG. 8 is a state diagram of the microwave oven using the flow diagramof FIG. 7;

FIG. 9 is the interrupt scheme used in conjunction with the softwareflow diagram of FIG. 7;

FIG. 10 is an alternate embodiment of the microprocessor and associatedhardware used in FIG. 5;

FIG. 11 is a side elevation view of the scale embodied in a bottom fedoven; and

FIG. 12 is a top view of the bottom fed oven of FIG. 11 taken alonglines 12--12.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a partially cut away microwave ovenhaving a heating cavity 10 containing a food body 12 positioned thereinthrough an access opening provided by a door (not shown). In the presentdescription, it is believed unnecessary to show and describe well knownand conventional parts such as, for example, the door seal structure. Itis preferable that microwave energy at 2450 MHz from a conventionalmagnetron 14 be coupled through waveguide 15 to a rotating primaryradiator 16 which has a pattern characterized in that a substantialportion of the energy is absorbed by the food before being reflectedfrom the walls of the cavity. More specifically, primary radiator 16comprises a two-by-two array of antenna elements 16a where each elementis an end driven half wavelength resonating antenna element supported bya length of conductor 16b perpendicular to the elements and the upperwall of the microwave oven cavity. Parallel plate microstriptransmission lines 16c connect each of the support conductors to acenter junction 16d axial to rotation. At the junction, a cylindricalprobe antenna 9 is attached to the radiator 16 structure. Probe antenna9 which has a capacitive hat 7 is supported by a plastic bushing 17positioned within the waveguide. The bushing permits rotation of theprobe antenna and radiator around the axis of the probe antenna.Microwave energy introduced into waveguide 15 by output probe 13 ofmagnetron 14 excites probe antenna 9. Energy couples down probe antenna9 which functions as a coaxial conductor through hole 19 in the upperwall of the oven cavity. The upper wall of cavity 10 is shaped to form adome 27 having a flattened conical shape extending outwardly in the wallto provide a nearly circular recess partially surrounding the directiverotating radiator and provides uniform energy distribution in theproduct being heated. The dome returns microwave energy reflected fromthe food body toward a circular area in the middle area of the microwaveoven cavity. It is preferable that air from a blower (not shown) used tocool the magnetron be circulated through the cavity to remove vapors. Itmay be preferable that this air be channeled into waveguide 15 andpassed through apertures 21 in the wall of the dome to provide rotationof radiator 16. Radiator 16 is connected to fins 23 to provide asuitable force surface for the air driven rotation. The fins may befabricated of a plastic nonlossy material. Other paths may also be usedto direct the air from the blower to the fins. Also, in lieu of the airdriven method, an electric motor (not shown) may be used to providerotation of the radiator. Grease shield 25 is transparent to microwaveenergy and provides splatter isolation from the rest of the cavity.

Control panel 30 which is shown in detail in FIG. 6 provides keyboardfunctions which are inputs to the control microprocessor 32 and displayfunctions by which the microprocessor indicates status to the user. Anyof a number of conventional keyboard switches and displays could beused. It may be preferable that well known capacitor touch pad switchesbe used for the keyboard. Also, it is preferable that the displayprovide digital read out of parameters such as time and a simultaneousindication of what keyboard entries have been selected. Specificfunctions of the control panel will be described in detail later herein.

Positioned below the floor 18 of the cavity is a scale 20. The scale hasfour vertical support pins 22 which respectively protrude through holes24 in the floor of cavity 10 in the proximity of the corners. Supportedon the pins is plate 26 which rests approximately one inch above thefloor of the cavity at the corners. Typically, the plate is made of apyrex glass material which is transparent to microwaves. The microwavespass through the glass, strike the floor of the cavity and are reflectedback up into the food body from the bottom side. This allows themicrowave energy to enter the food body from all sides. Also, the platemay provide some protection for the magnetron if the oven isaccidentally turned on when there is no load in the cavity. Although theglass plate may be removed for cleaning, it should always be in the ovenduring operation. The weight of the glass plate and any food bodies anddishes placed thereon is transferred through support pins 22 to scale20.

It is desirable that substantially no microwave energy pass through thefour pin holes 24 into chamber 28 below the cavity which houses thescale. Accordingly, the pin holes 24 which may preferably be circular,are less than one half wavelength in circumference. More specifically,the holes are slightly larger than the pins which are approximately onequarter inch in diameter. To minimize inaccuracies in scale weighings,it is important that there be as little friction as possible for a pinas it moves up and down through a hole; this may be accomplished byselecting tolerances that accurately position the pins to be concentricwith their respective holes and by using materials that have lowcoefficients of friction. It is preferable that the pins be fabricatedof a microwave transparent material such as a ceramic to provide amicrowave choke through the holes. If a pin were metallic, the structurewould exhibit the properties of a coaxial line with the outer conductorbeing the surface of the hole and the center conductor being the pin.Microwave energy would pass even though the size of the outer conductorwas below cutoff.

Scale 20 comprises four rigid lever arms 36. Each lever arm has aninverted V-bracket 37 on one end to support the arm from a knife edgedfulcrum 40. At the other end, each arm is attached to a second arm by asemicircular pivot pin 41 so that there can be vertical motion at thejoint of the arm pair between the fulcrums at the opposite ends. Thepairs of lever arms 36 so described are positioned parallel to eachother so that each arm of the pair has a corresponding arm in the otherpair. The corresponding arms are rigidly attached by a V shaped crossbar 43 running perpendicular to the connected lever arms. In thepreferred embodiment, each arm is approximately seven inches long andthe cross bars which are fourteen inches long are attached approximatelyone inch from the fulcrums. The scale was designed with these dimensionsso that it would fit in chamber 28 and the pins would protrude throughholes 24 at appropriate places. The compliant member 44 which resistsdownward motion of the lever arms at the pivot pin 41 joint is aflexible metal strip that is supported in cantilever fashion from block46. Rod 48 is attached rigidly and perpendicular to one of the leverarms near the pivot pin joint. The rod has a disk 50 on the end whichrests on compliant member 44.

As described earlier herein, the weight of plate 26 and any objectsplaced thereon is transferred to scale by pins 22 which protrude intothe cavity through holes 24 in the bottom cavity wall. Pins 22 areattached to rectangular brackets 52 which limit the upward movement ofthe pins through holes 24. The rectangular brackets 52 are rigidlyconnected at inside bottom points of V-shaped cross bars 43 adjacent tothe respective lever arms. Regardless of the distribution of downwardforce between the four pins 22, the force is transferred inapproximately the same ratio by the cross bars to the lever arms on thecompliant member side of the scale. Rod 48 couples the force from thelever arms through disk 50 to the compliant member 44. As the weight andcorresponding downward force is increased, the flexible compliant memberbends more; the compliant member is analogous to a spring. The verticalposition of the unsupported end of the compliant member is therefore afunction of the weight exerted on pins 22. The unsupported end ofcompliant member 44 is bent downward to form a shade member 57 thatshields a particular portion of light beam 54 from being incident onlight sensitive device 56. As the weight on plate 26 is increased sothat the unsupported end of compliant member 44 bends further downward,a greater portion of the light beam is blocked from being incident onlight sensitive device 56. Light sensitive device 56 may preferably be aphototransistor which provides an analog voltage which is a function ofthe light incident upon it. The source 58 of the light beam 54 may be alight bulb as shown or more preferably a light emitting diode as shownin the alternate embodiment of FIG. 4. It may be preferable to positiona concave lens between the source of light and the light sensitivedevice to focus the beam of light to a relatively small area.Accordingly, the intensity within that area would be varied rather thanvarying the area of light incidence.

Referring to FIG. 4, an alternate embodiment of the compliant member isshown. The light source 58 which may preferably be a light emittingdiode is attached to the unsupported end of compliant member 44 which isattached in cantilever fashion to block 46. The light beam 54 isdirected towards light sensitive device 56. A shade member 57a ispositioned between light source and the light sensitive device. As adownward force is exerted on the compliant member by rod 48 through disk50, an increased portion of the light beam is blocked by shade member57a. Accordingly, as the weight exerted downward on pins 22 isincreased, the analog voltage at the output of light sensitive device 56is decreased. A different type of shade member may preferably be usedwhich would block the upper portion of the light beam from beingincident on the light sensitive device. In this case, as more weight isplaced on the scale causing the beam to point further downward, agreater part of the beam would be incident on the light sensitive devicebecause it is not blocked by the shade member. This would mean that theoutput voltage from the light sensitive device would increase as afunction of increasing weight on the scale.

Scale 20 provides a means for providing microprocessor 32 with an inputindicative of the weight of objects in cavity 10. A substantialadvantage of scale 20 so described is that it can be installed incommercially available microwave ovens without significant retooling.More specifically, in the particular microwave oven to which the scalewas embodied, chamber 28 had a height of 3/8 inches in the center andapproximately 11/2 inches at the corners and edges. FIGS. 1 and 2 havenot been drawn to scale. The corners and edges of the floor 18 of cavity10 have always been raised so that a food body supported on plate 26would be elevated from the conductive surface of the floor wheredielectric losses would be very low. The scale which has a height ofapproximately one inch has its structure in a rectangular shape withnothing in the center so that it fits around the perimeter of chamber 28where the height is approximately 11/2 inches. Furthermore, becausethere is no structure in the center of the scale, it can be adapted foruse in a bottom fed microwave oven as will be described later hereinwith reference to the alternate embodiment of FIGS. 11 and 12.

Referring to FIG. 5, there is shown a block diagram of a microwave ovenembodying the invention. Scale 20 provides an input indicative of theweight of the food body to microwave processor 32. Using the weight ofthe food body along with other input parameters, the microprocessordetermines the output profile of the magnetron in time and power andcontrols the operation thereof.

Still referring to FIG. 5, the analog voltage output from the lightsensitive device of scale 20 is coupled to multiplexer 60 which operatesunder control of microprocessor 32. The function of multiplexer 60 is toprovide the microprocessor with a means of selecting which of aplurality of analog inputs is provided to analog to digital converter 62for conversion to a digital signal that is acceptable for input to themicroprocessor. An example of another analog input is from aconventional microwave temperature probe.

The reference clock for microprocessor 32 is provided by clock 64.Conventionally, clock 64 comprises an AC filter connected to the 60 HzAC power line and a zero crossing detector, the output of which iscoupled to the microprocessor.

Referring to FIG. 6, there is shown an expanded view of control panel 30of FIG. 1 which panel comprises keyboard 63 and display 65. As statedearlier herein, it may be preferable that the keyboard switches beconventional capacitive touch pad switches. Typically, a touch panelinterface may be connected between the keyboard and the microprocessor;the interface is of conventional design and is included in manycommercially available microwave oven models. Similarly, a high voltagedriver interface may be connected between the microprocessor anddisplays of control panel 30 to provide lighted indicators. The keyboardincludes touch pads 69 numerically labeled 0-9, functionally labeledCLOCK, READY TIME, DISH WEIGHT, THAW, WARM, HEAT, COOK PROGRAM, STIRTIMER, REDUCED POWER, TIMER, and push switches 67 labeled START,STOP/RESET and LIGHT. The display includes digital read outs 66,function indicator lights 68 associated with functionally labeled touchpads, and digital read out 70 associated with the COOK PROGRAM functionpad.

In operation, touch pads labeled 0-9 may generally be usedconventionally to enter data for well known functions into themicroprocessor. For example, when the microwave oven is not being used,digital read outs 66 display the time of day. To change the time of day,the user pushes numerical pads corresponding to the desired time; thistime is displayed in digital read outs 66. Then, when the user pushesCLOCK, the displayed time is entered into the microprocessor and becomesthe new time of day. Another example is to use the numerically labeledpads to display the amount of time food is to be cooked. Upon pushingSTART, the display time counts down until the over shuts off. The THAWfunction pad is used to activate the microprocessor to control themagnetron so that the food is raised from frozen food at 0° F. to thawedfood at 40° F. The WARM function pad is used to activate themicroprocessor to control the magnetron so that the food is raised from40° F. to 65° F. The HEAT function pad is used to activate themicroprocessor to control the magnetron so that the food is heated from65° F. to 160° F. The COOK PROGRAM function pad is used to activate themicroprocessor to control the magnetron so that the food at 160° F. istaken through the cooking process which may or may not raise itstemperature to above 160° F. In other words, the THAW, WARM, HEAT andCOOK inputs are indicative of the initial temperature of the food.Before initiating cooking, the COOK PROGRAM which is appropriate for theparticular food being cooked may be selected by touching an appropriatenumerical pad and then touching COOK PROGRAM. The selected program isdisplayed in digital read out 70. When in a cook-by-weight mode whichwill be described in detail herein, the REDUCED POWER pad may be touchedto activate TEMP HOLD which decreases the duty cycle of the magnetron.The 1/2, 1/4 and 1/8 indicators are activated by successive touchings ofthe REDUCED POWER pad during conventional cook-by-time operation. TheREADY TIME function pad is used to program the microwave oven to come onat a future time. The STIR TIMER is used to sound an alarm and shut offthe oven at a time when the food is to be stirred or other action takenwithin the oven. The TIMER function is used as a count down clock to analarm for timing which may or may not be associated with the microwaveoven. The START button initiates execution of a particular selectedprogrammed subroutine which turns the magnetron on. TheSTOP/RESET buttoncauses the magnetron to be turned off. Successive pushings of the LIGHTbutton causes a light (not shown) illuminating the cavity to be turnedon and off.

The use of microprocessors to control microwave ovens has become commonin the last decade. In fact, most if not all of the industry leadersoffer top-of-line microwave ovens that are microprocessor controlled. Ingeneral, the microprocessor receives inputs from a keyboard and sensorsand provides output signals which control the magnetron and drive thedisplay. In FIG. 5, a new sensor which is a scale coupled to weighobjects within the cavity has been added. However, the selection of anappropriate microprocessor and the programming of it to performspecified functions is well known to those skilled in the art. The earlymicroprocessor controlled ovens typically used standard commerciallyavailable processor integrated circuits and the application program wasprovided in a read only memory (ROM); these systems generally requiredmany input/output components to interface the microprocessor to thesystem. These interfaces are well known to those skilled in the art. Inrecent years, a continuing trend within the microwave oven industry isto use customized integrated circuits for controlling microwave ovens.The large volume of these specialized integrated circuits has enabledthe suppliers to spread the engineering development costs oftransforming user requirements to circuits over a large number ofintegrated circuits thus reducing the cost of the individual unit.Furthermore, there is a continuing trend to integrate more functionsonto a single silicon integrated circuit eliminating many of thediscrete electronic components and interface hardward such as digit andsegment drivers, analog to digital converters, multiplexers, zerocrossing detectors, AC filters, and touch panel interfaces. With theforegoing as a background and turning again to FIG. 5, microprocessor 32in the preferred embodiment is a customized integrated circuit developedby well known techniques and furnished by any one of a number ofelectronic suppliers. The integrated circuit has interface functionsintegrated into it. Even multiplexer 60 and analog to digital converter62 could have been included in the microprocessor integrated circuitsuch that analog signals are connected directly to the chip. Analternate embodiment of microprocessor 32 will be given later herein.

Still referring to FIG. 5, microprocessor 32 receives inputs from scale20 and keyboard 63 of control panel 30. In addition to performing manyconventional functions such as, for example, cooking for a set time,cooking at a set power, monitoring a temperature probe, and monitoringan interlock, microprocessor 32 performs a new function which relates tosimplified user operation. More specifically, the microprocessor usesthe weight of the food as weighed within the cavity along with theinitial temperature of the food to determine how long the food should becooked.

Referring to FIGS. 7, 8 and 9, software flow diagrams for theprogramming of microprocessor 32 in accordance with the invention areshown. Many conventional functions such as monitoring an interlock arenot included in the discussion herein but the inclusion of them in theflow diagrams and the programming of microprocessors from flow diagramsin general is well known to those skilled in the art. Referring first toFIG. 7, after POWER UP, the microprocessor is initially RESET whichincludes a number of conventional software house-cleaning proceduressuch as initialization of output channels. The following equation isused to CALCULATE HEATING TIMES.

    Heating Time=([HUS][FW+(DW)(SHD)]/([OPL][PLS][CF]

where HUS is Heat Units Selection, FW is Food Weight, DW is Dish Weight,SHD is Specific Heat of Dish, OPL is Oven Power Level, PLS is PowerLevel Selection and CF is Coupling Factor.

The first term in the heating time equation is Heat Units Selectionwhich is expressed in BTUs per pound of food. It has been found that therequired heat units per weight unit of food is in part a function of thetemperature range over which the food is to be heated and chemicaland/or physical changes taking place within the food. By a verysimplified user input from the keyboard, this term of the equation isdetermined. More specifically, referring again to FIG. 6, the userindicates the initial temperature state of the food by touching THAWwhich as labeled is for frozen foods (0° F.), WARM which as labeled isfor cold foods (40° F.) such as out of the refrigerator and/or HEATwhich as labeled is for food at room temperature (65° F.). Touching ofmore than one of these pads initiates a separate cycle for each functionand a separate calculation of the heating time equation for each cycle.For the THAW cycle, 100 BTUs per pound is entered into the equation; forthe WARM cycle, 25 BTUs per pound is entered into the equation; for theHEAT cycle, 100 BTUs per pound is entered into the equation; and for theCOOK cycle, 25-250 BTUs per pound is entered into the equation dependingon the COOK PROGRAM that is selected by touch pads and that is displayedwithin the COOK PROGRAM touch pad. Although the Heat Units Selectionentry into the equation for COOK determines the heating time for amaximum power level, that time will be increased by a specific factor ifa REDUCED POWER setting is selected. In other words, the same number ofBTUs for the cooking task are delivered but over a longer period of timefor more delicate cooking or simmering.

The second term in the heating time equation is [Food Weight+(DishWeight) (Specific Heat of Dish)]. The presence of the Food Weight in theequation is obvious; the multiplication of its units (pounds) by theunits of Heat Units Selection (BTU per pound) yields BTUs for thenumerator of the equation which when divided by the units (BTUs perminute) of the denominator, gives the quotient in minutes which are thedesired units. The inclusion of (Weight Dish) (Specific Heat of Dish) isto compensate for a certain portion of the heat which is provided to thefood being transfered to the dish by conduction. In other words, moreheat must be delivered to the food than might be thought necessarybecause some of it is lost by conduction to the dish. For usersimplicity, the specific heat of the dish in the calculation of theheating time equation is assumed to be a constant of 0.2 for the WARMand HEAT cycles where the temperature of the dish is raised byconduction as the temperature of the food rises. For the THAW and COOKcycles, the specific heat of the dish is set equal to zero to eliminatethe product of it and dish weight from the equation; with THAW, the BTUstransferred to raise the temperature of the dish is insignificantcompared to the BTUs to thaw the food and with COOK, which starts at160° F., there is no appreciable rise in temperature. Although a moreexacting expression of the heat lost by the food (and accordingly theadditional heat required to be delivered to it) would also include thespecific heat of the food and heat rise in gases in the cavity,empirical analysis has showed that the assumptions were adequate forproper operation of the oven using the heating time equation. Inoperation, when the light indicator on the DISH WEIGHT pad is on, it isindicative that a dish weight is stored in the microprocessor.Therefore, to commence a new cooking process with a new dish, the DISHWEIGHT pad is touched and the light indicator goes out; this erases theprevious dish weight from the microprocessor memory and "zeros thescale". The weight of the dish may then be set up for entry into themicroprocessor by either entering it through the numerical touch pads ifit is known or by placing the dish without food in the oven where itdepresses the scale. With a second touching of the DISH WEIGHT pad, theindicator light thereon goes on indicating that the new dish weight hasbeen entered into the microprocessor. It may be preferable that theanalog voltage at the output of light sensitive device 56 be somewhatlinear with the weight that is placed on the scale. With this being thecase, a linear analog to digital converter properly scaled can be usedso that the microprocessor directly samples weight in pounds. If theanalog voltage is not linear with weight such as being inverselyproportioned as the embodiment of FIG. 1, it can be compensated for inthe microprocessor by such conventional techniques as a lookup table.For accuracy of weighing, it may be preferable that at a weighing time,the microprocessor take a plurality of weight samples, discard high andlow weights, and average the remainder of the weights. The weight of thefood is calculated by the microprocessor by using the weighingimmediately prior to the START button being pushed and subtracting theweight of the dish after zero adjustment.

The first term in the denominator of the heating time equation is OvenPower Level. In the microprocessor calculation, it has been assumed thatthis value is a constant of 725 watts or 41.2 BTUs per minute. In actualoperation, there is generally an error in this assumption. Even ovens ofthe same model and manufacturer will typically vary over a range of 100watts from unit to unit. It is this inconsistency of output power thathas caused producers of prepared foods to specify in the microwavecooking directions on the box that microwave processing times may vary;this is true even though the characteristics of the food product arewell defined and can easily be empirically determined. Furthermore,output power may vary as a function of the AC line voltage. The error inthe assumption of 725 watts as the output power can be minimized byattempts to normalize ovens to that value.

The second term in the denominator of the heating time equation is PowerLevel Selection. If the REDUCED POWER pad has not been used to selectTEMP HOLD, a value of 1 is used for PLS in the heating time equation. Ifthe REDUCED POWER pad has been used to select TEMP HOLD, 0.3 plus 0.04per pound of food is input to the equation. For example, if the foodweighs 1 pound, the magnetron will operate at 34 percent of full power.Further, if the food weighs 2 pounds, 38 percent of full power will beoutputted. This is implemented by decreasing the duty cycle of themagnetron. In the past, it was generally accepted that just as somefoods cook better conventionally at lower rather than highertemperatures, some foods cook better at reduced microwave energy powerlevels. Accordingly, most microwave ovens provide many power levelselections. As part of the development of the cook-by-weight process, itwas found most important to determine the total number of BTUs requiredfor the particular food and then deliver them; however, the rate atwhich microwave energy is supplied is not so critical. In fact, the TEMPHOLD feature provides only one reduced power level setting and that is afunction of the weight of the food. Generally, the reduced power of TEMPHOLD is used to best advantage with food having a large volume where themicrowave energy penetration to the center of the food is greatlyreduced. Additional cooking time may be desirable to permit heat in theouter portion of the food to conduct toward the center for more uniformheating and cooking. It has been found that the most appropriate reducedpower setting is one which holds the food at temperature which forlightweight foods is approximately 30 percent of full power. Theadditional 4 percent per pound in the PLS formula compensates for largerfood bodies having greater surface areas and therefore greater heatlosses that must be compensated for to maintain temperature. Theassumption that food surface and size generally relates to weight hasbeen empirically tested.

The last term of the heat time equation is Coupling Factor. Not all ofthe microwave energy output from the magnetron is coupled into the food.Some of the energy is lost in the system such as in the walls,waveguide, and the plate. The percent of total energy (assumed to be 725watts) that is converted into heat in the food is in part a function ofthe food surface area and its absorptivity. For example, if one potatotakes four minutes to cook, two potatoes will generally take less thaneight minutes or twice that. This is because as the load is increased, alarger percentage of the total power is absorbed by the food. It hasbeen found that the distribution of energy into the food with respect tolosses is approximately expressed by the following formula.

    Coupling Factor=[Food Weight/(Food Weight+K)]

In essence, the constant K can be viewed as losses of the oven expressedin terms of weight. Constant K has been assigned the value of 0.1.Accordingly, if the food weighs 0.1 pounds, the coupling factor is onehalf or the heating time is increased by a factor of 2 over which itwould have otherwise been. If, however, the food weighed 1.0 pounds, theheating time would only be increased by a factor of 1.1. In FIG. 5,microprocessor block 32 indicates that the heating time per weight unitdecreases as a function of increasing weight because of the improvedcoupling of microwave energy into the greater food mass.

The discussion of the calculation of the heating time equation hasassumed that certain keyboard entries such as data relating to initialtemperature of the food and sensor entries such as the weight of thefood be available to the microprocessor. The required information whichis provided before the initial calculation and is periodically updatedis provided by interrupts as shown in FIG. 9. At the 60 Hz linecrossings and at the time midpoints therebetween as indicated by the 8.3MICROSECOND DELAY, the processor is interrupted at which time itDISPLAYS PARAMETERS, LIGHTS DISPLAY, SCANS KEYBOARDS and SELECTS A/DCHANNEL. At these times, the present keyboard data and scale data in themicroprocessor memory is updated.

Referring again to FIG. 7, after the calculation of the heating timeequation for the specific operational parameters, the program branchesat IS STATE ACTIVE? The active state is defined in FIG. 8 which showsthe relationship between oven states. More specifically, after power up,the microprocessor automatically goes to a reset state as describedearlier with reference to FIG. 7. Next, the microprocessor automaticallygoes to the idle state wherein the heating time equation is continuallycalculated. The microprocessor stays in the idle state until the startbutton of control panel 30 is pushed. At that time, the microprocessorgoes to an active state where it stays until either the stop button ispushed or the cooking function is finished. If the stop button ispushed, the microprocessor goes to a hold state from which it can returnto the active state by pushing the start button 90 or back to the resetstate as initiated by a second pushing of the stop button. Againreferring to FIG. 7, if the state is not active, the heating timecalculation is executed again. If the state is active, the next questionis ANY COOK-BY-WEIGHT FUNCTIONS? These functions as described withreference to control panel 30, are THAW, WARM, HEAT and COOK. If none ofthese has been selected but the processor is active, it is indicativethat the function is cook-by-time. If one or more of the weightfunctions has been selected, the software initially tests to see IS THAWFLAG SET? If it has, the processor controls the magnetron which iscycled on and off, the cycle duty time being a function of the weight ofthe food. More specifically, the THAW function as described earlierprovides magnetron on-time to supply 100 BTUs per pound to the food. Thepower level is always 100 percent. If the food weighs less than 3pounds, 100 BTUs per pound is provided with an equivalent off timebefore the next function. If the food weighs 3 or more pounds but lessthan 10 pounds, the same 100 BTUs per pound is supplied but inincrements of 25 BTUs per pound with time intervals therebetween equalto the time required to supply 50 BTUs per pound. If the food weighs 10or more pounds, it is the same as the previous sentence except the offtime intervals are equal to the time to supply 75 BTUs per pound. Also,the flash thaw indicator on control panel 30 is flashed. As describedwith reference to FIG. 9, this action would be taken at the 60 Hz linecrossings and the midpoints therebetween. Also, the thaw time wouldcount down. At the end of the thaw cycle or if no thaw cycle wasselected, the microprocessor determines IS WARM FLAG SET? If it has, themicroprocessor turns on the magnetron, flashes the warm indicator andcounts down until the end of the warm cycle. At the end of the warmcycle or if no warm cycle was selected, the microprocessor determines ISHEAT FLAG SET? If it has been, the microprocessor turns on themagnetron, flashes the heat indicator and counts down until the end ofthe heat function. At the end of the heat cycle or if no heat cycle wasselected, the microprocessor determines IS COOK FLAG SET? If it hasbeen, the microprocessor turns on the magnetron, flashes the cookindicator and counts down until the end of the cook cycle. At thecompletion of this cycle or if the cook flag was not set, the softwareflow returns to the reset subroutine.

Again referring to FIG. 5, the microprocessor, using techniques wellknown to those of ordinary skill in the art, controls the power supply68 for the magnetron 14. When the cooking time is finished, themicroprocessor controls power supply 71 to shut off magnetron 14. If thecooking time is to be performed at reduced power, the microprocessorregulates the duty cycle of the magnetron. The microprocessorsimultaneously provides a visual indication on display 65 of the timefor the magnetron to be on and what functions have been selected.

The invention provides a significant advance in the microwave heatingart in that it is a major step towards one button simplified operation.Many former problems associated with the user determining cookingparameters have been overcome. The weight of the food which is providedautomatically by a scale in the oven is entered into a microprocessorwhich is programmed to calculate the proper cooking time and thencontrols the magnetron in addition to giving an operational status tothe user through displays.

Referring to FIG. 10, an alternate embodiment of the circuit of FIG. 5is shown. As described earlier herein, for commercial applications, itmay be preferable that the microprocessor control be provided by acustomized integrated circuit which includes therein many of theinterface functions. The embodiment of FIG. 10 shows a general purposemicroprocessor with ancillary hardware and interfaces coupling it to themicrowave oven control panel, sensors, and magnetron control. An exampleof microprocessor 100 that could be used is a MOS Technology, Inc.MCS6502. As shown in FIG. 10, the microprocessor is connected to databus 102 which typically comprises eight lines which may be connected toMCS6502 pins 26-33, respectively. The microprocessor is also connectedto address bus 104 which typically comprises sixteen lines which may beconnected to MCS6502 pins 9-25, respectively. Conventional initiatingcircuitry (INIT) 106 is used only at power up time by the microprocessorand may be connected to input pins 6 and 40 of microprocessor MCS6502.Further, a conventional crystal clock (CLOCK GENerator) 108 is requiredand may be input to the microprocessor on pins 37 and 39. Line 110 isused to provide the clock to peripheral interface devices 112 and 114,program memory (ROM) 116 and data memory (RAM) 118. Microprocessor 100provides the same functions as the microprocessor described withreference to FIG. 5. The program memory 116 which preferably is a readonly memory stores the operational program. The task of writing theprogram from the requirements given herein with reference to FIGS. 7, 8and 9 are well known to one skilled in the art. Microprocessor 100provides addresses to address bus 104 to fetch instructions from programmemory 116 and data from data memory 118 which is a random accessmemory. Write enable and other control functions are provided frommicroprocessor 100 to data memory 118 or peripheral interface devices112 and 114 on control bus 120.

Peripheral interface devices 112 and 114 allow microprocessor 100 toread data from keyboard 63, to test the state of sensors and switches,display the results of internal operations and control the magnetron.Example peripheral interface devices 112 and 114 are MCS6522's which mayhave pins 21-40 connected to control, timing, interrupt, data bus andaddress bus. Peripheral interface device 112 provides interface forcontrol panel 30 which includes keyboard 63 and displays 65. Keyboardinputs to the microprocessor are provided by a conventional matrix scantechnique. More specifically, the keyboard comprises a matrix ofswitches which may be of the contact or capacitive touch variety. Forthe control panel of FIG. 6, a 4×6 matrix would be sufficient; however,a larger matrix will be described and it is assumed that it may containfunctions not discussed herein. Output signals are sequentially providedto the columns of the matrix and the rows are sensed and decoded. Indetail, pins 10-17 of MCS6522 are connected to eight lines 124 connectedto high current output buffer 126 and segment output port 128. At theoutput of high current output buffer 126 which may, for example, be a74LS374, eight lines 130-138 as indicated connect through eightamplifiers 139 to the keyboard. Sequence column scanning pulses areprovided on lines 130-138; the rows of the matrix of switches of thekeyboard are sensed by lines 140 which are connected to pins 1-9 ofperipheral interface device 112. The sensed data is decoded wherebymicroprocessor determines which switches of the switch matrix ofkeyboard 63 are closed.

Digital displays 65 are scanned which means that each digit is drivenfor a short period of time, such as two milliseconds, in sequence. Theentire display is scanned at a rate which the eye cannot detect. Lines130-138 are coupled through driver circuits, two circuits in FIG. 6being representative of eight in the embodiment. Each conventionalcircuit as shown comprises Vcc which is typically +5 volts, R1 which maybe 1.5K ohms, R2 which may be 1.0K ohms, and transistor Q. Thesesequenced driver circuits determine which digit of the display isactivated. The data that determines which segments of a particular digitare on is determined by the output of segment output port 128 which iscoupled to lines 142-150 through resistors R3 to displays 65. An exampleof a segment output port is an MC3482. The data and scan pulses timeshare lines 124, the enable control to port 128 and buffer 126 beingprovided on lines not shown by peripheral interface device 114 on pins 3and 4, respectively.

Microprocessor 100 controls the output of magnetron through peripheralinterface device 114. More specifically, outputs from peripheralinterface device 114 on lines 160 are connected to high current outputbuffer 162 which may be, for example, a 74LS374. As shown in FIG. 10,two of the outputs of buffer 162 are connected to conventional opticalisolators 164 and 166 which may be, for example, MOC3010s. A LOW voltage(logical 0) at the input of an optical isolator causes the internalresistance of its output to be a short circuit.

In response to a control signal from optical isolator 164, triac 168 isturned on energizing filament transformer 173. In response to a controlsignal from optical isolator 166, triac 169 is turned on energizing highvoltage power supply 171 which typically comprises a regulatingtransformer in accordance with well known practice. In operation,filament transformer 173 energizes the filament of magnetron 73 and highvoltage power supply 171 provides approximately 4000 volts to the plateof the magnetron.

Omitted from FIG. 10 are many common features such as, for example,interlocks, a blower and fuses. Light emitting diode 180 which is partof scale 20 directs light towards photodetector 182. As described withreference to the preferred embodiment, the analog voltage output of thephotodetector is preferably substantially linear with the weight of thefood in the microwave cavity. The analog voltage output on line 184 istransferred to analog to digital converter 186 which upon command fromthe microprocessor through peripheral interface 114 on line 188 providesa pulse output which has a time duration determined by the analogvoltage input. Information derived from this pulse is transferredthrough peripheral interface device 114 to microprocessor 100 on databus 102. By counting the duration of the pulse, microprocessor 100determines the weight on the scale.

Referring to FIGS. 11 and 12, respective side and top views show scale20 embodied in a bottom fed microwave oven such as a conventionalmicrowave electric range. Electric heating element 200 is positionedtowards the bottom of cavity 202 of a microwave electric oven. Themicrowave energy is provided by magnetron 204 which has an output probe206 which is inserted directly into well 208 of the cavity. Themicrowave energy couples from the output probe 206 through a directiveradiator 210 having three antenna ports 212. The microwave energypropagates through microwave transparent cover 214. Choke structure 216prevents microwave energy from leaking out of the gap between well 208side walls and the floor of the cavity. A blower 218 directs air acrossthe fins of magnetron 204 and up into well 208 by duct 220 throughapertures 222. The flow of air may be used to provide a force on veins224 of radiator 210 to provide rotation. Scale 20 which is the same asthat described with reference to FIGS. 1, 2 and 3, is supported bybrackets 230 extending downward from the oven floor as shown in FIG. 11.Because scale 20 substantially forms a rectangle without structure inthe interior, the bottom fed microwave source can be positioned in themiddle of the oven floor without structural interference. Pins 22protrude through holes in the floor of the oven to support plate 26. Thepins 22 in this embodiment are longer than the embodiment of FIGS. 1, 2and 3 so as to rise aove electric heating element 200. Pins 22 may alsoprovide support for oven racks so as to provide a weight indication offood bodies placed on them.

This concludes the description of the preferred embodiment. The readingof it however will bring to mind many modifications to one skilled inthe art without departing from the spirit and scope of the invention.Accordingly, it is intended that the scope of the invention be limitedonly by the claims.

What is claimed is:
 1. A microwave oven comprising:an outer housing; aconductive cavity within said housing, said cavity having asubstantially rectangular floor; a chamber in said housing below saidfloor; a weight sensing device positioned in said chamber; said floorhaving four apertures respectively located in regions adjacent to thefour corners, said apertures having a perimeter of less than one-halfwavelength of said microwave energy; four microwave transparent verticalcolumns supported by said weight sensing device and respectivelyprotruding through said four apertures in said floor into said cavity; aplate positioned in said cavity and supported by said four columnswhereby the weight of an object placed on said plate is coupled to saidweight sensing device; said weight sensing device providing a firstsignal which is a function of the weight supported by said columns; amicroprocessor; a control panel having operator selectable means forproviding a second signal corresponding to the initial temperature ofsaid object; a magnetron for supplying microwave energy to said cavity;and said microprocessor controlling said magnetron in response to saidfirst and second signals.
 2. The oven recited in claim 1 wherein saidweight sensing device comprises a compliant scale.
 3. A microwave ovencomprising:an outer housing; a conductive cavity within said housing,said cavity having a substantially rectangular floor; a chamber in saidhousing below said floor; a weight sensing device positioned in saidchamber; said floor having four apertures respectively located inregions adjacent to the four corners, said apertures having a perimeterof less than one-half wavelength of said microwave energy; fourmicrowave transparent vertical columns supported by said weight sensingdevice and respectively protruding through said four apertures in saidfloor into said cavity; a plate positioned in said cavity and supportedby said four columns whereby the weight of an object placed on saidplate is coupled to said weight sensing device; said weight sensingdevice providing a first signal which is a function of the weightsupported by said columns; a microprocessor; a control panel havingoperator selectable means for providing a second signal corresponding tothe initial temperature of said object; a magnetron supplying microwaveenergy to said cavity; and said microprocessor in response to said firstand second signals determining the exposure time of microwave energy toraise said initial temperature to a predetermined higher temperature,and controlling said magnetron in accordance with said exposure time. 4.The oven recited in claim 3 wherein said weight sensing device comprisesa compliant scale.